The binary size increases from 49184 to 49216.

Start with PATH_MAX bytes and then double each time.

Issue reproduction steps:

1. Spawn a GUI program e.g. open a PDF file in zathura or evince.
2. Without quitting `nnn` close the terminal.
3. Notice that the application quits too.

Can be a nagging issue if someone is not using a drop-down terminal.

Fix: detach a GUI child and start it in a new session.

There are 2 aspects to this commit:

- It fixes #81: in cases where we do not wait for a spawned child we can assume
that the child is a GUI process. We detach and spawn the child in a new session.
- It changes the behaviour to @Rahi374's suggestion in PR #40 i.e. invoke the DE
opener in F_NOWAIT mode.

This reverts commit 7be0726164.

This reverts commit 7654a2e0c6.

This reverts commit 30fdb6ff11.

Sizes like 1.999 get rounded to (1 + 100/100), which should be displayed
as 2.00 not 1.100 as simple %i.%02i would do without additional measures.

Use distinct (by username) tmp filename to copy file paths.

The pattern used is:
/tmp/nnncp$USER

If username is 'arun', the file name is `/tmp/nnncparun`.

In nav-as-you-type mode only control keys are understood when filter is enabled.
We need some way to quicly jump HOME, last visited dir or start dir.